< draft-newman-telnet-sasl-00.txt		draft-newman-telnet-sasl-01.txt >
	Network Working Group C. Newman		Network Working Group C. Newman
	Internet Draft: Using SASL/GSSAPI with Telnet Innosoft		Internet Draft: Telnet SASL Option Innosoft
	Document: draft-newman-telnet-sasl-00.txt January 1998		Document: draft-newman-telnet-sasl-01.txt November 1998
	Using SASL and GSSAPI with Telnet		Telnet SASL Option
	Status of this memo		Status of this memo
	This document is an Internet-Draft. Internet-Drafts are working		This document is an Internet-Draft. Internet-Drafts are working
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	Introduction		Abstract
	Telnet has its own custom authentication negotiation framework		It is common today for Internet client software to implement
	defined in ''Telnet Authentication Option'' [TELNET-AUTH]. This has		multiple Internet protocols. SASL [SASL] provides an
	primarily been used for Kerberos v4 [TELNET-KRB], but is largely		authentication framework which permits multi-protocol clients and
	unused otherwise as there is only limited development of telnet		servers to reuse security-sensitive authentication code. This memo
	products and protocols. It is desirable to have telnet leverage		defines a SASL profile for the Telnet [TELNET] protocol.
	development of new security services for new protocols. Therefore,
	future use of the Telnet authentication option is deprecated in
	favor of a new SASL [SASL] authentication option suitable for use
	with SASL and GSSAPI [GSSAPI] mechanisms. This service can
	complement use of TLS with Telnet [TELNET-TLS].
	[NOTE: This proposal is in response to a request by the TN3270e WG		This proposal will be discussed on the telnet-ietf mailing list.
	to have SASL or GSSAPI available when TLS is too heavy-weight. I		To subscribe, send the word "subscribe" to
	will not request a telnet option number for this until there is		<telnet-ietf-request@bsdi.com>.
	rough concensus that it is a good idea. Public discussion of this
	mechanism may take place on the tn3270e@list.nih.gov mailing list
	with a subscription address of listserv@list.nih.gov. Private
	comments may be sent to the author].
	1. Conventions Used in this Document		1. Conventions Used in this Document
	The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD		The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD
	NOT", and "MAY" in this document are to be interpreted as described		NOT", and "MAY" in this document are to be interpreted as described
	in "Key words for use in RFCs to Indicate Requirement Levels"		in "Key words for use in RFCs to Indicate Requirement Levels"
	[KEYWORDS].		[KEYWORDS].
	Formal syntax is defined using ABNF [ABNF].		In examples, "C:" and "S:" indicate data sent by the client (end
			which does a TCP active option) and server (end which does a TCP
	In examples, "C:" and "S:" indicate lines sent by the client and		passive open) respectively.
	server respectively.
	2. Mechanism Requirements
	It is important that all telnet implementations are capable of
	interoperable authentication without the use of unencrypted
	plaintext passwords.
	For the usage scenarios with the TN3270 protocol, it looks like the
	TLS protocol in combination with traditional embedded plaintext
	passwords will be preferable because TN3270 is often performed by a
	proxy with no knowledge of the users. This suggests it is the
	correct mandatory to implement mechanism for TN3270. Regular
	telnet servers, on the other hand, have been successfully modified
	to support mechanisms such as Kerberos, and a full TLS layer might
	be more expensive than necessary in many cases. This is where a
	lighter-weight SASL or GSSAPI mechanism may be preferable.
	Unfortunately, the CRAM-MD5 [CRAM-MD5] SASL mechanism (which is		2. Background for this Proposal
	likely to be the mandatory to implement choice for a number of
	services) is particularly unsuited to the Telnet protocol as a
	machine which supports both CRAM-MD5 and remote login risks
	exposing the authentication database and thus risks an attacker
	gaining the ability to impersonate all users to any CRAM-MD5
	authenticated service with the same passphrase.
	The SCRAM-MD5 [SCRAM] mechanism might suffice, but is probably too		Telnet has its own single-protocol authentication framework defined
	new to gain rough concensus. This leaves OTP-SHA1 [OTP-SASL] as		in "Telnet Authentication Option" [TELNET-AUTH] which predates SASL
	the preferred choice for the mandatory to implement lightweight		[SASL]. This old Telnet authentication option and the associated
	authentication mechanism in combination with this telnet extension.		encryption option [TELNET-ENC] do not provide integrity protection
			facilities, machine parsible error codes (e.g. to alert the client
			of an expired passphrase), or a way to integrate GSSAPI [GSSAPI]
			into Telnet.
	[NOTE: This issue is certainly open for debate, as is the wisdom of		Adding SASL and using it for new authentication mechanisms will
	replacing the old telnet AUTHENTICATE option].		improve reuse of security-sensitive code in multi-protocol clients
			in addition to addressing the other issues. While it would be
			possible to layer SASL support on top of the existing
			authentication option, it could result in an extra round-trip and
			would have potentially confusing interactions with the modifiers
			field in the Telnet authentication option.
	3. Kerberos V4 Compatibility		3. Kerberos Compatibility
	Both SASL and the old Telnet authentication option offer Kerberos		Both SASL and the old Telnet authentication option offer Kerberos
	V4 mechanisms. It is usually not desirable to deploy two		V4 mechanisms. It is usually not desirable to deploy two
	incompatible mechanisms for the same function, however, the		incompatible mechanisms for the same function, however, the
	KERBEROS_V4 SASL mechanism is more resistant to reply attacks and		KERBEROS_V4 SASL mechanism is more resistant to reply attacks and
	provides encryption services. Currently deployed Kerberos V4		provides integrity services. Currently deployed Kerberos V4 Telnet
	telnet implementations use encryption support which was documented		implementations have no integrity protection and the encryption
	in an expired internet draft and is susceptible to active attacks.		service is subject to an active down negotiation attack.
	Implementations which offer support for the KERBEROS_V4 SASL		Implementations which offer support for the KERBEROS_V4 SASL
	mechanism SHOULD also implement the old Telnet authentication		mechanism SHOULD also implement the old Telnet authentication
	option Kerberos v4 mechanism. This will provide better		option Kerberos v4 mechanism. This will provide better
	interoperability with deployed implementations. When both options		interoperability with deployed implementations. When both options
	are available, the KERBEROS_V4 SASL mechanism SHOULD be used in		are available, the KERBEROS_V4 SASL mechanism SHOULD be used in
	preference to the old telnet authentication mechanism.		preference to the old Telnet authentication mechanism, unless
			encryption without integrity protection is desired.
	The author is not aware of implementation of Kerberos V5 via the		Both the SASL GSSAPI mechanism and the old Telnet authentication
	old Telnet authentication option. Therefore the GSSAPI SASL		option offer Kerberos V5 mechanisms. The only difference is that
	mechanism is the preferred method for Kerberos V5. [NOTE: anyone		the SASL GSSAPI Kerberos 5 mechanism includes integrity protection
	else know of a Kerb5 telnet?]		not available via the old authentication and encryption options. A
			server supporting Kerberos V5 SHOULD implement the old Kerberos V5
			authentication option for backwards compatibility.
	4. SASL Telnet Option		4. SASL Telnet Option
	The SASL telnet option is telnet option number XXX. For historical		The GSSAPI/SASL service name for this profile of SASL is "rcmd".
	reasons, the GSSAPI/SASL service name for this SASL profile is
	"rcmd".
	#define TELOPT_SASL XXX		The SASL Telnet option is Telnet option number XXX. It has the
			following subnegotiation options:
	It has the following subnegotiation options:		LIST 0
			START 1
			STEP 2
			CANCEL 3
			DONE 4
	#define TELSASL_LIST 0		The DONE subnegotiation option has the following codes:
	#define TELSASL_START 1
	#define TELSASL_STEP 2
	#define TELSASL_SUCCESS 3
	#define TELSASL_FAIL 4
	The SASL telnet option is negotiated only one way. The server		SUCCESS 0
	offers the SASL option with "WILL SASL" and the client announces		CANCELLED 1
	support with "DO SASL." Once the option is successfully		BADAUTH 2
	negotiated, the server sends the LIST subnegotiation containing a		BADPROT 3
	space separated list of SASL mechanisms available:		NOTAUTHZ 4
			EXPIRED 5
			ENCRYPT 6
			TOOWEAK 7
			TRANS 8
			DISABLED 9
	S: IAC WILL SASL		The SASL Telnet option is negotiated only one way. The server asks
	C: IAC DO SASL		the client to use SASL with "DO SASL" and the client announces
	S: IAC SB SASL LIST "KERBEROS_V4 GSSAPI CRAM-MD5 OTP-SHA1" IAC SB		support with a "WILL SASL" message. Once the option is
	The client then sends the START subnegotiation to begin a SASL		successfully negotiated, the server sends the LIST subnegotiation
	exchange with the server. This subnegotiation contains the		containing an ASCII string with a space separated list of available
	mechanism name followed by an ASCII NUL character followed by the		SASL mechanisms:
	initial client response, if present. The client is not required to
	wait for the LIST message from the server prior to sending the
	START message.
	C: IAC SB SASL START "CRAM-MD5" NUL IAC SE		S: IAC DO SASL
			C: IAC WILL SASL
			S: IAC SB SASL LIST "KERBEROS_V4 GSSAPI CRAM-MD5 OTP" IAC SB
			The client sends the START subnegotiation to begin a SASL exchange
			with the server. The START subnegotiation contains the desired
			mechanism name optionally followed by an ASCII NUL character and an
			initial client response. The client is not required to wait for
			the LIST message from the server prior to sending a START message.
			C: IAC SB SASL START "CRAM-MD5" IAC SE
	This is followed by a series of STEP messages containing SASL		This is followed by a series of STEP messages containing SASL
	messages for the client and server respectively:		messages for the client and server respectively:
	S: IAC SB SASL STEP		S: IAC SB SASL STEP
	"<1896.697170952@postoffice.reston.mci.net>" IAC SE		"<1896.697170952@postoffice.reston.mci.net>" IAC SE
	C: IAC SB SASL STEP "tim b913a602c7eda7a495b4e6e7334d3890" IAC SE		C: IAC SB SASL STEP "tim b913a602c7eda7a495b4e6e7334d3890" IAC SE
	Note that it is important to perform IAC doubling if the octet		Note that it is important to perform IAC doubling if the octet
	value hexidecimal FF occurs in any SASL data. The server indicates		value 255 occurs in any SASL data. This applies to data in the
	successful completion of the exchange by sending the "SUCCESS"		START, STEP and DONE suboptions.
	subnegotiation which MAY contain an optional final mutual
	authentication step.
	S: IAC SB SASL SUCCESS IAC SE		When a client receives a STEP message from the server, it MAY
			cancel the authentication with the CANCEL message. The server will
			respond with a DONE CANCELLED message. If the client wishes to
			begin a new authentication, it MAY send a START message without
			waiting for the server DONE CANCELLED message.
			C: IAC SB SASL CANCEL IAC SE
			S: IAC SB SASL DONE CANCELLED IAC SE
			The server indicates successful completion of the exchange by
			sending the DONE subnegotiation with SUCCESS status, which MAY
			contain final server authentication data (usually for mutual
			authentication purposes).
			S: IAC SB SASL DONE SUCCESS IAC SE
	If a SASL security layer is negotiated, it begins on the server end		If a SASL security layer is negotiated, it begins on the server end
	immediately after the SASL SUCCESS subnegotiation, and begins on		immediately after the DONE SUCCESS subnegotiation, and begins on
	the client end immediately after the last client START or STEP		the client end immediately after the last client START or STEP
	subnegotiation once the SUCCESS subnegotiation is received.		subnegotiation once the SUCCESS subnegotiation is received.
	The server indicates failure by sending the FAIL message with an		For those cases where a security layer including integrity
	optional error code:		protection is negotiated, the server SHOULD send another LIST
			suboption message matching the one initially sent. If the client
			supports any stronger authentication mechanism, it SHOULD verify
			that the new LIST suboption matches the one sent prior to
			authentication.
	S: IAC SB SASL FAIL BADAUTH IAC SE		The server indicates failure by sending the DONE message with a
			code other than SUCCESS, followed by a human readable string in the
			character set currently active on the Telnet channel. If no
			character set has been negotiated through prior agreement or the
			Telnet CHARSET option [TELNET-CHARSET], then UTF-8 [UTF-8] is
			assumed.
	The following error codes are defined:		S: IAC SB SASL DONE BADAUTH "Authentication Failed" IAC SE
	#define TELSASL_BADAUTH 0 /* authentication failed */		The following error codes are defined by this specification. When
	#define TELSASL_BADPROT 1 /* protocol violation */		in doubt of the appropriate error code, the BADAUTH error code
	#define TELSASL_NOTAUTHZ 2 /* authorization failed */		should be used. Additional error codes MAY be defined by future
	#define TELSASL_EXPIRED 3 /* passphrase/credentials expired */		standards track or IESG approved experimental RFCs.
	#define TELSASL_ENCRYPT 4 /* encryption or stronger mech needed */
	#define TELSASL_TOOWEAK 5 /* mechanism too weak for user */		CANCELLED
	#define TELSASL_TRANS 6 /* transition needed to use new mech */		The authentication was cancelled by the client.
	#define TELSASL_DISABLED 7 /* account disabled */
	BADAUTH		BADAUTH
	This indicates that the user does not exist or the		This indicates that the user does not exist or the
	authentication failed for a reason other than those listed		authentication failed for a reason other than those listed
	below.		below.
	BADPROT		BADPROT
	This indicates the client attempted to use a mechanism not		This indicates the client attempted to use a mechanism not
	supported by the server, or the protocol for the SASL		supported by the server, or the protocol for the SASL
	mechanism was not followed.		mechanism was not followed.
	NOTAUTHZ		NOTAUTHZ
	This indicates the client successfully authenticated, but is		This indicates the client successfully authenticated, but is
	not authorized to login to the service with the requested SASL		not authorized to login to the service with the requested SASL
	authorization identity.		authorization identity.
	EXPIRED		EXPIRED
	This indicates that the client passphrase, one time passphrase		This indicates that the client passphrase, public key
	or public key certificate has expired and can be updated with		certificate or other credential has expired and can be updated
	an appropriate passphrase/credential change protocol.		with an appropriate passphrase/credential change protocol.
	ENCRYPT		ENCRYPT
	This indicates that the requested client mechanism is not		This indicates that the requested client mechanism is not
	permitted without an encryption layer, such as that provided		permitted without an encryption layer, such as that provided
	by TLS. The client may activate such encryption, or try a		by TLS. The client may activate such encryption, or try a
	stronger mechanism.		stronger mechanism.
	TOOWEAK		TOOWEAK
	This indicates that security policy does not permit the		This indicates that security policy does not permit the
	requested user to use the requested mechanism. For example,		requested user to use the requested mechanism. For example,
	an administrative user might be required to use a stronger		an administrative user might be required to use a stronger
	mechanism.		mechanism.
	TRANSThis indicates the user has a valid verifier in a server		TRANS
			This indicates the user has a valid verifier in a server
	authentication database but the requested mechanism can not be		authentication database but the requested mechanism can not be
	used with that verifier. This also indicates that if the		used with that verifier. This also indicates that if the
	client changes the passphrase or does a one-time		client changes the passphrase or does a one-time
	authentication with a plaintext passphrase mechanism		authentication with a clear-text passphrase mechanism
	(preferably encrypted), then the appropriate authentication		(preferably encrypted), then the appropriate authentication
	database for the requested mechanism will be initialized.		database for the requested mechanism will be initialized.
	DISABLED		DISABLED
	This indicates that the user's account has been disabled. The		This indicates that the user's account has been disabled. The
	user must contact a system administrator to get their account		user must contact a system administrator to get their account
	re-enabled.		re-enabled.
	10. Security Considerations		5. Formal Syntax
			The following formal syntax uses ABNF [ABNF]:
			IAC = %d255 ; standard Telnet symbols
			DO = %d253
			WILL = %d251
			SB = %d250
			SE = %d240
			SASL = %dXXX ; Telnet SASL option
			LIST = %d0 ; Telnet SASL sub-options
			START = %d1
			STEP = %d2
			CANCEL = %d3
			DONE = %d4
			; Miscellaneous single-character symbols
			DIGIT = %d30-39 ; US-ASCII digit character
			UPALPHA = %d65-90 ; Uppercase alphabetic characters
			MECH-CHAR = %d65-90 / DIGIT / "-" / "_"
			SAFE-DATA = %d0-254 ; octets which don't need quoting
			TEXT = %d1-254 ; human readable text
			SP = %d32 ; US-ASCII space
			NUL = %d0 ; US-ASCII NUL
			; Miscellaneous multi-character symbols
			quoted-255 = %d255 %d255
			sasl-mech = 1*20mech-char
			subopt-data = SAFE-DATA / quoted-255
			text = *subopt-data ; human readable text
			sasl-data = *subopt-data
			success = %d0 sasl-data
			error = %d1-254 text
			; Telnet SASL messages
			sasl-do = IAC DO SASL
			sasl-will = IAC WILL SASL
			sasl-list = IAC SB SASL LIST *(sasl-mech SP) sasl-mech IAC SE
			sasl-start = IAC SB SASL START sasl-mech [NUL sasl-data] IAC SE
			sasl-step = IAC SB SASL STEP sasl-data IAC SE
			sasl-cancel = IAC SB SASL CANCEL IAC SE
			sasl-done = IAC SB SASL DONE (success / error) IAC SE
			6. Security Considerations
	This inherits the security considerations of SASL [SASL] and any		This inherits the security considerations of SASL [SASL] and any
	underlying mechanism used.		underlying mechanism used.
	The SASL LIST subnegotiation is not integrity protected and is thus		The SASL LIST subnegotiation is not integrity protected and is thus
	susceptible to tampering by an active attacker. The client can		susceptible to tampering by an active attacker. There are two ways
	address this issue by having a configurable list of acceptable		to mitigate this attack: (1) have the client explicitly configured
	mechanisms. In addition, if a SASL integrity protection layer is		to use a specific mechanism and never fall back to a weaker one.
	negotiated on, the server SHOULD re-issue the SASL LIST		(2) have the client configurable to require integrity protection,
	subnegotiation after the integrity layer is active so the client		and verify that the LIST suboption value is the same both before
	has the option of checking for tampering. A client which supports		and after the integrity protection is applied.
	a weaker integrity protected mechanism and a stronger mechanism
	SHOULD verify the re-issued SASL LIST subnegotiation is unchanged
	if the weaker integrity protected mechanism is used.
	With some SASL mechanisms, the ENCRYPT or TOOWEAK error codes will		With some SASL mechanisms, the ENCRYPT or TOOWEAK error codes will
	be generated after sensitive information has been exposed. For		be generated after sensitive information has been exposed. For
	this reason, clients SHOULD be configurable to disable weaker		this reason, clients SHOULD be configurable to disable weaker
	mechanisms which might reveal sensitive information and SHOULD do		mechanisms which might reveal sensitive information and SHOULD do
	so for user, mechanism and server combinations which result in		so for user, mechanism and server combinations which result in
	these error codes.		these error codes.
	The TRANS error code could be spuriously generated by an active		The TRANS error code could be spuriously generated by an active
	attacker. For this reason, the client SHOULD NOT use a weaker		attacker. For this reason, the client SHOULD NOT use a weaker
	mechanism in response to a TRANS error code without explicit user		mechanism in response to a TRANS error code without explicit user
	permission. The TRANS error code can also be used to probe for		permission. The TRANS error code can also be used to probe for
	untransitioned users at a site. For this reason, sites must		untransitioned users at a site. For this reason, sites must
	consider the tradeoffs between a user-friendly transition to a		consider the tradeoffs between a user-friendly transition to a
	stronger mechanism and the risks entailed by permitting such		stronger mechanism and the risks entailed by permitting such
	transitions.		transitions.
	11. References		Telnet server and client implementations MUST check for buffer
			overrun on Telnet subnegotiations and deal with more data than will
			fit in an internal buffer gracefully.
	[ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF",		7. References
	RFC 2234, Internet Mail Consortium, Demon Internet Ltd, November 1997.
			[ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
			ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd,
			November 1997.
	[CRAM-MD5] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension		[CRAM-MD5] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension
	for Simple Challenge/Response", RFC 2195, MCI, September 1997.		for Simple Challenge/Response", RFC 2195, MCI, September 1997.
	[GSSAPI] Linn, "Generic Security Service Application Program		[GSSAPI] Linn, "Generic Security Service Application Program
	Interface, Version 2", RFC 2078, OpenVision Technologies, January		Interface, Version 2", RFC 2078, OpenVision Technologies, January
	1997.		1997.
	[KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Requirement		[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
	Levels", RFC 2119, Harvard University, March 1997.		Requirement Levels", RFC 2119, Harvard University, March 1997.
	[OTP-SASL] Newman, "One Time Password SASL mechanism", work in progress.		[OTP-SASL] Newman, C., "The One-Time-Password SASL mechanism", RFC
			2444, Innosoft, October 1998.
	[SASL] Myers, "Simple Authentication and Security Layer (SASL)", RFC		[SASL] Myers, "Simple Authentication and Security Layer (SASL)",
	2222, Netscape Communications, October 1997.		RFC 2222, Netscape Communications, October 1997.
	[SCRAM] Newman, "Salted Challenge Response Authentication Mechanism		[TELNET] Postel, J., Reynolds, J., "TELNET PROTOCOL SPECIFICATION",
	(SCRAM)", work in progress.		RFC 854, ISI, May 1983.
	[TELNET-AUTH] Borman, "Telnet Authentication Option", RFC 1416, Cray		[TELNET-AUTH] Borman, "Telnet Authentication Option", RFC 1416,
	Research, Inc., February 1993.		Cray Research, Inc., February 1993.
	[TELNET-KRB] Borman, "Telnet Authentication: Kerberos Version 4", RFC		[TELNET-CHARSET] Gellens, R., "TELNET CHARSET Option", RFC 2066,
	1411, Cray Research, Inc., January 1993.		Unisys, January 1997.
	12. Author's Address		[TELNET-ENC] Ts'o, T., "Telnet Data Encryption Option", work in
			progress.
			[TELNET-KRB] Borman, "Telnet Authentication: Kerberos Version 4",
			RFC 1411, Cray Research, Inc., January 1993.
			[UTF-8] Yergeau, F. "UTF-8, a transformation format of ISO 10646",
			RFC 2279, Alis Technologies, January 1998.
			8. Author's Address
	Chris Newman		Chris Newman
	Innosoft International, Inc.		Innosoft International, Inc.
	1050 Lakes Drive		1050 Lakes Drive
	West Covina, CA 91790 USA		West Covina, CA 91790 USA
	Email: chris.newman@innosoft.com		Email: chris.newman@innosoft.com
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